virt: Fix a bug in RemoteFile init

If remote file doesn't exist, SCPTransferFailedError
will be catched, and then creat a new empty file om
local host.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: add truncate() func into RemoteFile class

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #1693 from liyangfnst/remote_file

virt: Fix 2 bugs in RemoteFile class